This invention relates to an improved hand-off technique for a wireless network. More particularly, the invention relates to use of a known messaging sequence, e.g., ISANSWER messaging in the IS-41 protocol, in a heretofore unknown manner to improve hand-off procedures that may be established between a serving (anchor or intermediate) Mobile Switching Center (MSC) and a target (or border) mobile switching center. That is, the ISANSWER message is selectively sent from the serving (anchor or intermediate) mobile switching center to the target (or border) mobile switching center during the hand-off process that may be induced during call termination to a mobile station. The transmission of the ISANSWER message in this manner allows the target (or border) mobile switching center to end ring back to a caller and to actually connect the call when the mobile station answers the call while it was still within a cell area of the serving (anchor or intermediate) mobile switching center.
While the invention is particularly directed to the art of hand-off techniques, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications.
By way of background, a variety of hand-off techniques are well known in the wireless industry. Many of these hand-off techniques are governed by the American National Standards Institute Standard 41 (ANSI41 or IS-41).
To illustrate one type of hand-off technique, referring to FIG. 1, a network 10 includes a first mobile switching center 12 and a corresponding cell site 14. Also illustrated in FIG. 1 are a second mobile switching center 16 and a second cell site 18. It should be appreciated that the first mobile switching center 12 may also be referred to as the serving (anchor or intermediate) mobile switching center and the second mobile switching center 16 may also be referred to as the target (or border) mobile switching center. This nomenclature is useful because, as illustrated, a mobile device 20 migrates from an area of cell site 14 corresponding to the mobile switching center 12 to an area of cell site 18 corresponding to the mobile switching center 16. The anchor MSC is the first to assign a traffic channel. In the examples, a simple scenario of a handoff from the serving MSC or anchor MSC is described. Nevertheless, note that the handoff, as well as the concept of using ISANSWER, may occur in other situations. For example, the handoff may also happen from an intermediary MSC (after handoff from anchor), still during a call termination. Also shown in FIG. 1 is another communication device 24 which, for purposes of this example, may take a variety of forms and is operative to establish a call between itself and the mobile device 20.
Using the example network of FIG. 1, a hand-off procedure embodied in a call flow 30 is illustrated in an example in FIG. 2. The call flow 30 begins with a hard hand-off request being transmitted from the cell site 14 to the serving (anchor or intermediate) mobile switching center 12 (at line 32). This occurs after a mobile device, such as the mobile device 20, migrates from an area corresponding to, for example, the cell site 14 (and the signal there becomes weak) to an area corresponding to, for example, the cell site 18.
The serving (anchor or intermediate) mobile switching center 12 then sends an invoke message, e.g., a Facility Directive (FACDIR2) message, to the target (or border) mobile switching center 16 (at line 34). It should be understood that the functionality of providing ring back to a caller that may be calling the mobile device may be (and in many actual systems is) provided from the target (or border) mobile switching center after this message is received by this target (or border) MSC. So, from this point forward, ring back is provided by the target (or border) mobile switching center, not the serving (anchor or intermediate) mobile switching center.
The rationale for this transfer of ring back responsibility to the target (or border) mobile switching center is based on the fact that, when the mobile device is answered on the target (or border) MSC, the system can simply perform normal switching functions through, for example, a Digital Cellular Switch (DCS). The ring back function can then simply be discontinued. This discontinuation of ring back is immediate. Otherwise, if the serving (anchor or intermediate) mobile switching center were providing the ring back, there would be a delay between the actual answer on the target (or border) and the receipt and processing of the ISANSWER invoke message (conventionally only sent from the target (or border) mobile switching center to the serving (anchor or intermediate) mobile switching center). This delay could result in overlap of “talking” phase of the call and giving the ring back to the caller.
Referring back to FIG. 2, the target (or border) mobile switching center 16 then sends a (FACDIR2) return result to the serving (anchor or intermediate) mobile switching center 12 (at line 36). The mobile switching center 12 consequently sends a message to the mobile device 20 through cell site 14 to start the handoff process, like, if applicable, to retune to an appropriate frequency so that the hand-off can be successfully completed (at line 38).
During a successful hand-off, the cell site 18 will ultimately send a hand-off confirmation message to the target (or border) mobile switching center 16 (at line 40). Then, the mobile switching center 16 completes the hand-off procedure by sending a mobile-on-channel message to the serving (anchor or intermediate) mobile switching center 12 (at line 42).
If the mobile device 20 is called by another communication device (such as the communication device 24 in FIG. 1) during hand-off, and the mobile device 20 is answered while in the area of cell site 18, then the cell site 18 sends an answer message to the target (or border) mobile switching center (at line 44). The target (or border) mobile switching center 16 then sends an ISANSWER invoke message to the serving (anchor or intermediate) mobile switching center (at line 46) to indicate that the call has been answered. Of course, the target (or border) mobile switching center 14 also ends the ring back to the caller at this point in time and connects the call by connecting the traffic channel from (the mobile and) the target cell to the inter-vendor trunk between target and serving MSCs.
The network as illustratively shown in FIG. 1 does not, however, address the situation where the mobile device 20 is answered after FACDIR2 invoke is sent while still in the area of the cell site 14 during the hand-off procedure. At present, if this situation occurs, the ring back will continue to be provided by the target (or border) mobile switching center 16 and the caller using communication device 24 will continue to receive a ring back, as opposed to being connected to the mobile device 20. This is, of course, not a desirable situation.
In particular, during the handoff process from the serving (anchor or intermediate) mobile switching center to the target (or border) mobile switching center, there is a “time window” of interest. This time window begins when the FACDIR2 invoke message is sent from the serving (anchor or intermediate) mobile switching center to the target (or border) mobile switching center. The time window ends when the mobile device actually re-tunes from the cell site associated with the serving (anchor or intermediate) mobile switching center to the cell site associated with the target (or border) mobile switching center. The time window is of a significant, non-zero, length. During this time window, two IS-41 messages are typically received at the serving (anchor or intermediate) mobile switching center from the target (or border) mobile switching center. In this scenario, however, there is no message sent from the serving (anchor or intermediate) mobile switching center to the target (or border) mobile switching center after the FACDIR2 invoke message. Therefore, there is no mechanism that is able to direct the target (or border) mobile switching center to stop the ring back (and switch the call), if the mobile device answers within the area of the serving (anchor or intermediate) mobile switching center (i.e., within the area of cell site 14) during the “time window” specified above. Thus, the target (or border) mobile switching center will keep ringing back towards the caller, even after the called mobile device answers. Plus, the appropriate switching action on the target (or border) MSC connecting the mobile to the trunk between the target (or border) and the serving (anchor or intermediate) MSCs may not happen. This is clearly unacceptable, as this would interfere with the conversation that could otherwise start at this point.
The present invention contemplates a new and improved hand-off technique for a wireless network that resolves the above-referenced difficulties and others.